Chapter XIII 



EVIDENCE FROM PROTOZOANS THAT SUB- 

 STANCES OTHER THAN CHROMATIN ARE 

 PHYSICAL BASES OF HEREDITY 



TAKING it as proved that in most sexually propagating 

 organisms heredity is dependent on chromosomes, thus 

 making the view that chromosomes are bearers of heredity 

 legitimate in a certain sense, a fundamental question must 

 be examined before the discussion can be regarded as hav- 

 ing even an approach to comprehensiveness. This question 

 may be stated thus : is heredity dependent on the chromo- 

 somes alone, that is, to the exclusion of other parts of the 

 cell; in other words, are chromosomes the sole "bearers of 

 heredity"? 



At the outset of this inquiry we must recall what heredity 

 is as understood in this treatise. It is resemblance between 

 living beings due to descent. This is the definition which in 

 an earlier section we decided is more satisfactory than any 

 other when due consideration is given not only to the phe- 

 nomena of organic propagation themselves, but also to the 

 historic usage of the word. Another thing about heredity 

 insisted upon on earlier pages should be recalled: all stages in 

 the development of an individual are as truly manifestations 

 of heredity as is the final or adult stage. And finally, the 

 reader is asked not to forget the deprecation expressed 

 early in the discussion of the unwarrantable practice with 

 many recent writers on heredity of eitlier ignoring asexual 

 propagation altogetlier or tossing it aside as presenting no 

 problem or anything of significance to the geneticist. 



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